1. Field of the Invention
This invention relates to a method for molding polyurethane seat pads for use as automotive seats, and more particularly, to a method for molding polyurethane seat pads having a center seating portion and bank or raised portions on opposite sides thereof or along the periphery thereof having a higher hardness than the center seating portion.
2. Prior Art
In these days, seats for automobiles and other vehicles are often manufactured by furnishing polyurethane seat pads each having a center seating portion and a pair of bank or raised portions on opposite sides thereof and covering the pads with shells or covers. Some polyurethane seat pads have uniform hardness throughout, but polyurethane seat pads in which the bank portions are harder than the seating portion are more popular for their stable support to the human body and the improved aesthetic appearance of finished covers. Especially preferred is a seat pad in which only the outer peripheral zone of the bank portion is made of high rigidity material and the inner zone of the bank portion surrounded by the outer peripheral zone, that is, the inner zone of the bank portion surrounding and defining the seating portion is made of relatively soft material. Since the portion of the seat pad which comes in direct contact with the human body is soft enough, this seat pad gives no uncomfortable feel to the person to sit on and ensures improved supporting and holding ability and improved outer appearance.
Prior art methods for preparing such seat pads include an insert method involving the steps of burying in a foaming polyurethane liquid a separate insert in the form of a high rigidity polyurethane foam such as chip molded parts and semi-hard parts and causing the liquid to expand to form an integral foam having the insert buried therein; and another method involving the steps of preforming a collecting vessel from expanded polystyrene, placing the collecting vessel in a mold, and pouring in the collecting vessel a foaming polyurethane liquid capable of forming polyurethane having higher rigidity as disclosed in Japanese Patent Publication (JP-B) No. 811/1992 or U.S. Pat. No. 4,923,746.
These methods, however, have several drawbacks. The former method must separately mold a high rigidity insert and thus significantly adds to the cost. In the latter method, thermal contraction of expanded polystyrene is essential for the expanded polystyrene to bond with urethane foam. Since thermal contraction of expanded polystyrene abruptly takes place in a very narrow temperature range and excessive contraction can cause deformation of the collecting vessel itself, a careful temperature control is required, leaving a problem of production yield. In addition, since expanded polystyrene is susceptible to attack by organic solvents, a highly polar solvent such as methylene chloride contained in the foaming polyurethane liquid as a foaming agent can attack the collecting vessel of expanded polystyrene to form holes or cause substantial deformations, failing to obtain products with good outer appearance.
To overcome these problems, the inventors previously proposed in Japanese Patent Application Kokai (JP-A) No. 261410/1991, USSN 07/623,689, now U.S. Pat. No. 5,244,612 or EP 431981A2 a method for molding a polyurethane seat pad having a center seat portion and bank portions on opposite sides thereof or along the periphery thereof using a mold defining a cavity having a center region corresponding to the center seat portion and side regions corresponding to the bank portions, comprising the steps of: placing collecting vessels of expanded polyolefin in the side regions of the mold cavity, pouring a foaming polyurethane liquid for producing a low hardness foam into the cavity between said collecting vessels, pouring a foaming polyurethane liquid for producing a high hardness foam into said collecting vessels, and causing the foaming liquids to expand, thereby producing a polyurethane seat pad having the collecting vessels embedded therein.
This molding method overcame the above-mentioned problems since the expanded polyolefin is more resistant against heat and chemicals than expanded polystyrene. In addition, that side of the bank portion facing the seating portion is formed of urethane foam. There is thus obtained a seat pad which is comfortable to sit on.
Our previously proposed molding method using expanded polyolefin as collecting vessels involves placing the collecting vessels in the side regions of the mold cavity and pouring a foaming polyurethane liquid into the collecting vessels secured in the mold cavity still has the problem that if the collecting vessels themselves have less flexural strength, they cannot be fixedly secured within the mold at a desired position or they can be deformed, deflected or even drop down from the secured position by the weight of the foaming polyurethane liquid admitted therein, and they can be changed in shape by the pressure or heat exerted upon foaming of the foaming polyurethane liquid, failing to produce seat pads of desired configuration. That is, there are sometimes obtained seat pads which have varying hardness in the side or bank portions. This problem might be overcome by increasing the thickness of expanded polyolefin or by reducing the foaming rate, but the resulting side or bank portions providing an uncomfortable feel.
The expanded polyolefin left in the cured polyurethane foam, which is essentially poor in adhesion to the polyurethane, apparently shows an integral structure, but tends to delaminate through repetitive compression during long term use as the seat. Such delamination causes the seat pad to change rigidity in its side or band portions and gives a negative impact on the comfortable feel to sit on since the collecting vessels are felt as foreign matter.